Being a Montana native, I can tell you that the general population of the larger cities (which are mostly in the mountains... even Billings being on the edge) do not consider tornadoes much of a threat as they don't happen too often or in high visibility areas. Most Montana tornadoes are typically to the north and east where the land is much more flat. Perhaps that explains the apparent complacency of the drivers. I have a lot of friends in Billings and one told me that she had told her daughter not two days before that tornadoes don't happen in the area.
As an aside, this tornado case is interesting to me because of the topography. I've spent a lot of time in Billings and the area that the tornado formed is basically a pass or canyon where the Yellowstone river flows through higher terrain. To the north and west are the Rimrocks, basically a cliff that lines the north edge of the city. To the south and east are another set of cliffs, bluffs if you will, adjacent to the Yellowstone.
The storm formed and was rather stationary for a while. I wonder if tornadogenesis was aided by the inflow coming up the Yellowstone river valley (out of the ENE) while rain cooled air drained into the canyon from WSW. Another interesting thing is that the 88D is near the airport, atop the Rims and though quite close to the tornado, may have been scanning fairly high into the storm even at that proximity.
